1. Field of the Invention
The present invention relates to an image forming method of correcting skew of a sheet picked up from a sheet retaining unit before forming an image on this sheet, and an image forming apparatus employing this method, and, more particularly, to an image forming method of surely accomplishing skew correction and an image forming apparatus employing this method.
2. Description of the Related Art
Image forming apparatuses, such as a copying machine, a printer and a facsimile, employ a latent image forming type recording apparatus like an electrophotographing apparatus, due to a recent demand for image recording on normal sheets of paper. According to this image forming principle, after a photosensitive drum as a latent image carrier is precharged, the photosensitive drum is exposed to a light image to have an electrostatic latent image formed thereon. This electrostatic latent image is developed by a developing unit so that a toner image is formed on the photosensitive drum. This toner image is then transferred onto a sheet of paper.
In this image forming apparatus, sheets on which an image is to be formed are retained in a sheet cassette, and should be picked up therefrom for a later imaging process. The sheets may be skewed when being picked up from the sheet cassette. When an image is formed on a skewed sheet, the image is also formed askew. To prevent it, the image forming apparatus is provided with a mechanism of correcting skew of the picked-up sheet.
FIGS. 1A and 1B are explanatory diagram of prior art, the former showing the structure of the prior art and the latter presenting a time chart of the prior art.
As shown in FIG. 1A, a sheet is picked up by a pickup roller 92 from a sheet cassette 91 retaining a plurality of sheets, and abuts against resist rollers 93. As the leading edge of the sheet abuts against the resist rollers 93, the sheet is bent and tends to return to the original form. This restoring force eliminates the skewing of the sheet. Thereafter, the resist rollers 93 are rotated to feed the sheet forward. A toner image formed on a photosensitive drum 90 of the image forming mechanism is transferred onto the sheet by a transfer roller 94.
In this image forming apparatus, the image forming mechanism starts the image forming operation in accordance with the pickup of the sheet. The image forming apparatus therefore needs a mechanism for detecting the sheet pickup. Because foreign matter such as sheet powder is likely to be produced and a developer scatters in the apparatus which forms a toner image, an inexpensive and small photosensor cannot be used as the sheet detecting mechanism.
Accordingly, a mechanical sheet sensor BS is used as the sheet detecting mechanism. According to the prior art, this sheet sensor BS is provided at the subsequent stage of the resist rollers 93 to detect a sheet feed through the resist rollers 93 so that the supply of the sheet to the image forming mechanism is checked. That is, as shown in FIG. 1B, when the pickup roller 92 is driven for a given time, a sheet abuts against the resist rollers 93 and skewing is thus considered as corrected. Then, the resist rollers 93 are driven to feed the sheet while the photosensitive drum 90 is rotated, and image writing by a laser beam starts, thus forming an image.
At this time, if the sheet sensor BS does not detect any sheet within a given period of time after the driving of the resist rollers 93, the rotation of the resist rollers 93 and photosensitive drum 90, which have been driven, is stopped as indicated by the broken lines in FIG. 1B.
But, the skew correction by the resist rollers 93 needs a constant and stable amount of bending of sheets (resist amount). Conventionally, this resist amount is determined by the expected amount of rotation of the pickup roller 92 on the premise that the picked-up sheet always abuts against the resist rollers 93. If the pickup roller 92 and a sheet slip on each other or a similar incident occurs at the time of sheet pickup, therefore, the resist amount of the sheet becomes non-uniform or insufficient so that the sheet may be fed forward without skew correction.
Further, since non-arrival of the sheet at the resist rollers 93 due to sheet pickup failure or the like is detected by the sheet sensor BS, such detection cannot be accomplished before the driving of the resist rollers 93 and the photosensitive drum 90. This means the requirement of wasteful driving of the photosensitive drum, which is one factor to shorten the service life of the photosensitive drum.